A variety of implantable medical devices for delivering a therapy and/or monitoring a physiological condition have been clinically implanted or proposed for clinical implantation in patients. Some implantable medical devices deliver electrical stimulation to, and/or monitor conditions associated with, the heart, muscle, nerve, brain, stomach or other organs or tissue. Some implantable medical devices employ one or more elongated electrical leads carrying stimulation electrodes, sense electrodes, and/or other sensors. Implantable medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of stimulation or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead, which may be implanted at the desired location. A proximal portion of the lead may be coupled to an implantable medical device housing, which may contain circuitry such as signal generation and/or sensing circuitry.
Some implantable medical devices, such as cardiac pacemakers or cardioverter-defibrillators, provide therapeutic electrical stimulation to the heart via electrodes carried by one or more implantable leads. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion, or defibrillation. In some cases, an implantable medical device senses intrinsic depolarizations of the heart, and controls delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing pulses to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation.
Implantable medical leads typically include a lead body containing one or more elongated electrical conductors that extend through the lead body from a connector assembly provided at a proximal lead end to one or more electrodes located at the distal lead end or elsewhere along the length of the lead body. The conductors connect signal generation and/or sensing circuitry within an associated implantable medical device housing to respective electrodes or sensors. Some electrodes may be used for both delivery of therapeutic signals and sensing. Each electrical conductor is typically electrically isolated from other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Medical lead bodies implanted for cardiac applications tend to be continuously flexed by the beating of the heart. Other stresses may be applied to the lead body, including the conductors therein, during implantation or lead repositioning. Patient movement can cause the route traversed by the lead body to be constricted or otherwise altered, causing stresses on the lead body and conductors. In rare instances, such stresses may fracture a conductor within the lead body. The fracture may be continuously present, or may intermittently manifest as the lead flexes and moves. Also, the wear and degradation of the insulation between the conductors may result in shorting.
Additionally, the electrical connection between medical device connector elements and the lead connector elements can be intermittently or continuously disrupted. For example, connection mechanisms, such as set screws, may be insufficiently tightened at the time of implantation, followed by a gradual loosening of the connection. Also, lead pins may not be completely inserted.
Lead fracture, disrupted connections, or other causes of short circuits or open circuits may be referred to, in general, as lead related conditions. In the case of cardiac leads, sensing of an intrinsic heart rhythm through a lead can be altered by lead related conditions. In particular, lead related conditions may cause noise in a cardiac electrogram signal received by an implantable medical device, which may be incorrectly sensed by the implantable medical device as cardiac beats.
This phenomenon is referred to as oversensing, i.e., oversensing of cardiac beats. Other causes of oversensing include other causes of non-physiologic noise in the cardiac electrogram, such as electromagnetic interference, and incorrectly characterizing a T-wave within the cardiac electrogram as a cardiac beat, which is referred to as T-wave oversensing. Oversensing may cause an implantable medical device to incorrectly detect a cardiac tachyarrhythmia, and deliver an unnecessary shock to treat the cardiac tachyarrhythmia.